Abstract : This paper proposal a method of fault diagnosis for low-speed mini-motor with acoustics signals under the motor is not disassembling condition. Due to there are two uncertain events between fault of motor and acoustics signals of motor. These events will be described by fuzzy logic. A new probability concept of fuzzy events will apply in the method. The condition of mini-motor running will be described by membership grade set of feature value of acoustics signals. By using the inference method a new membership set will result. The new membership set not only can judges the motor quality, but also can recognize the kind of motor failure. The high frequency spectrum energy of acoustics signals and low frequency spectrum energy of acoustics signal is applied in the method as optimal feature value. Actual experiment results and numerical simulation's results all show the method of the combination fuzzy theory is very effectively method for fault diagnosis of low-speed mini-motor.